This invention relates to a method and apparatus for detecting air cavities, such as pneumothorax and pneumoperitoneum, in humans and animals. Pneumothorax is the state in which air or other gas is present in the pleural cavity and which occurs spontaneously as a result of disease or injury of lung tissue or puncture of the chest wall. Pneumoperitoneum is the state in which air or other gas is present in the peritoneal cavity.
Pneumothoraces are commonly encountered as a spontaneous process, an iatrogenic complication, or secondary to traumatic injuries. Air pressure in the pleural space may increase leading to tension pneumothoraces requiring prompt diagnosis and treatment. Pneumothorax diagnosis often requires radiographic confirmation since history and physical examination findings can be non-specific. Valuable time may be lost while waiting for chest x-ray results and even after they are obtained, interpretation can be uncertain or incorrect.
Spontaneous pneumoperitoneum (free air in the abdomen) results from a hole in the wall of the GI tract. Detection in the setting of acute abdominal symptoms is important since most such cases require emergency surgery. The most common causes of pneumoperitoneum are perforated gastric or duodenal ulcer or colonic perforation secondary to mechanical obstruction, infection, infarction, severe ileus, ulcerated carcinoma or trauma.
Gastrointestinal perforation (GIP) is a common condition resulting from either trauma or progression of ulcerating, inflammatory, ischemic, or mechanically obstructing diseases of the gastrointestinal (GI) tract. It is estimated that there are from 10,000 to 70,000 cases in the United States each year. Much higher rates of GIP would be expected in regions of armed conflict or poor medical care. High morbidity or mortality rates accompany those abdominal catastrophes, as spillage of microbial, enzyme and other intraluminal contents into the peritoneum typically cause rapid disease advancement and often death if proper initiation of medical and surgical treatment is delayed. Ready access to a low cost and safe technology that would immediately identify GIP would save many lives-each year. Currently, GIP is diagnosed preoperatively by imaging of free intraperitoneal air.
Several techniques exist to diagnose GIP and pneumoperitoneum, including radiographs, computerized tomography (CT) examination and ultrasound. Each technique has limitations of availability, cost or accuracy. For example, meticulously performed plain radiographs with the patient positioned in the upright or left lateral decubitus (left side down) positions for ten to twenty minutes reportedly can detect small amounts of intraperitoneal gas. However, it is uncommon for ill patients with acute abdominal pain to be kept in those positions, at least for more than a brief period. Therefore, supine (lying on the back) radiographs are the most commonly obtained tests for pneumoperitoneum. Although a recent review suggests several ways to improve diagnostic accuracies, typical pneumoperitoneum detection sensitivities are less than 60%.
Although availability, cost and time delays may limit utility, CT examination is currently the most sensitive and specific tool for diagnosing intraperitoneal gas. Careful studies demonstrate that CT is capable of reliably detecting even minute amounts of air. The superiority of CT is striking when compared to a sensitivity of only 38% for upright radiographs. The accuracy of ultrasound imaging may be similar to plain radiography, but more studies are required to confirm its precise utility. Ultrasound and CT scanning, though accurate, are more expensive than radiographs and often unavailable in a timely manner. This is especially true in remote areas, such as rural regions, battlefield settings or in developing nations.
Researchers have applied the technique of external low-frequency vibro-acoustic excitation and response measurements to the diagnosis of other biological conditions. For example, Wodicka et al., xe2x80x9cSpectral Characteristics of Sound Transmission in the Human Respiratory System,xe2x80x9d IEEE Transactions of Biomedical Engineering, Vol. 37, No. 12, December 1992, pp. 1130-35, Wodicka et al., xe2x80x9cTransfer function of sound transmission in subglottal human respiratory system at low frequencies,xe2x80x9d The American Physiological Society, 1990, pp. 2126-2130, and V. Goncharoff, xe2x80x9cWideband acoustic transmission of human lungs,xe2x80x9d Med. and Biol. Eng. and Comput., 27, 1989, pp. 613-619 have studied the acoustic transmission properties from the trachea to the chest wall. They found that sound transmission times were frequency dependent as different wavelengths of sound coupled to different parts of the lung lining, principally due to geometrical changes. While these studies offer some relevant information regarding types of indices which can be used for analysis and types of transducers, there are many issues specific to the abdomen as opposed to the chest and lungs due to the great differences in their structures.
Several researchers have led efforts in the utilization of low frequency vibro-acoustic excitation, i.e., 20 to a few hundred Hertz, coupled with doppler ultrasonic imaging, which is sometimes referred to as sonelastic imaging. This technique has been proposed to locate tumors which produce significant changes in stiffness properties in an otherwise acoustically homogeneous region. The presence of the tumor, or localized stiffness, will distort the resonant shapes of vibration patterns caused by low frequency excitation. These patterns can be imaged using the very expensive laser doppler vibrometry.
Accordingly, there is a need for an accurate, low cost, portable technology capable of diagnosing GIP, pneumothorax and pneumoperitoneum with minimum discomfort to the patient.
To achieve the foregoing and other objects and in accordance with the principles of the invention, a low cost, painless and safe method and apparatus for diagnosing patients with gastrointestinal perforation is described.
The underlying physics principles employed in the invention are similar to those used during chest percussion in which the hyperresonance often associated with large pneumothoraces is a manifestation of bioacoustic changes. Low frequency vibro-acoustic properties of the abdomen depend on the abdominal contents and free (extraluminal) air produces measurable differences in the vibro-acoustic response. Thus, if known excitations are applied to the abdomens of perforated patients, response differences are detectable by a vibro-acoustic sensor.
An apparatus for detecting the presence of a gas cavity in the thorax, abdomen, peritoneal cavity and elsewhere in a body includes an actuator for transmitting a source of vibro-acoustic waves into a first location. The actuator introduces a standardized audible sound, gently, into the chest wall. A white noise generator producing vibro-acoustic waves in the range of 5 Hz to 2000 Hz is generally desirable. Electromagnetic shakers and speakers may also be used in place of the actuator. A detector or acoustic sensor, such as an air-coupled microphone (electronic stethoscope), is placed at a second location for detecting the transmitted vibro-acoustic waves. The detector detects changes in the chest wall caused by the presence of an air cavity and generates a signal representative of the frequency response of the chest cavity. Preferably, the actuator and detector are positioned on the body at locations effective for detecting the suspected air cavity. For a supine subject, this would be in the most anterior position. The level of the third rib may be chosen in human subjects to avoid the diaphragm. Indeed, during the detection phase, the operator can move the detector to different locations to test for the largest peaks (resonance) and dips (anti-resonance) in the response signal. A processor analyzes the frequency response of the detected signal for the presence of resonance waves and anti-resonance waves and other acoustic property changes, which are indicative of chest cavity changes. A gas cavity is detected when the frequency response shows a peak, indicative of a resonance wave, followed by a dip, indicative of an anti-resonance wave.
Several types of detectors (transducers) may be used: vibro-acoustic sensors, microphones, air-coupled microphones and optical detectors. For optimal coupling to the skin surface, the measurement sensor""s dynamic impedance should match that of the skin surface. It has been found that lower signal to noise ratios were observed for air coupled sensors at high frequencies. The response of air coupled microphones was found to be sensitive to the size and geometrical shape of the coupling surface. Impedance matched accelerometers have been used in place of microphones in some studies of the, abdominal region.